1. Field of the Invention
The present invention relates to a heating apparatus comprising a detection system for detecting high temperature water vapor emanating from an object and controlling a heat source by using the detection signal.
2. Description of the Related Art
A heating apparatus having a system for detecting automatically a finished state of a heated object finds applications in various forms. A humidity sensor for detecting humidity changes is most widely used as a detector for the detection system of such an automatic heating apparatus. The humidity sensor is used to detect changes in electrical resistance of an element due to the water molecules adsorbed on the surface thereof. In order to prevent the deterioration in sensitivity due to the smear of the element surface and to maintain a stable performance over a long period, it is necessary to burn off the smear from the element surface or take any other complicated procedure at regular intervals of time.
As disclosed in U.S. patent application No. 429,286, on the other hand, the inventors are studying a system in which the water vapor or other vaporized substance of high humidity emanated from an object with the heating thereof is collected by way of a vent formed in the wall of a heating chamber and is applied against a pyroelectric element outside of the heating chamber to detect a finished state of heating through a voltage generated from the pyroelectric element. This system is based on a physical phenomenon of a detection mechanism exchanging heat between the pyroelectric element and the vapor, and therefore unlike in conventional humidity sensors, the sensitivity would not be substantially affected by the smear of the element surface, thereby leading to the advantage of constructing a detection system in a very simple manner in principle.
The disadvantage of a system which utilizes temperature changes of the pyroelectric element caused by the heat of vapor is that the pyroelectric element would be undesirably energized to generate a voltage not only by vapor generated from an object but also by a high-temperature air, that is, hot air applied suddenly thereto. In the case of a microwave oven comprising an electric or gas heater as a secondary heat source other than microwaves, a hot air of the secondary heat source remains in a great amount immediately after a heating operation. If an object is heated with the microwave oven under this condition, the pyroelectric element responsive to the residual hot air would generate a voltage regardless of the temperature of the food and thus fail to discriminate the voltage due to the vapor emanated by the heating of the food, thereby leading to an erroneous detection.
Also, this problem is liable to arise after a long heating operation with microwaves alone, as well as after heating with an auxiliary heater of a microwave oven, because of a similar phenomenon similar to the one mentioned above due to an increased temperature of the heating chamber or the like, thus making it difficult to detect a heated condition of the object food (a finished condition by heat) with high accuracy without error.
With the increase in the temperature of the pyroelectric element, the smaller temperature difference with the vapor generated from the food reduces the detection sensitivity. This variation of detection sensitivity according to operating conditions makes it difficult to secure stable detection accuracy.